Large-scale extended emission around the Helix Nebula: Dust, molecules, atoms, and ions

We present new observations of the ionized gas, molecular gas, and cool dust in the Helix Nebula (NGC 7293). The ionized gas is observed in the form of an Halpha image, which is constructed using images from the Southern Halpha Sky Survey Atlas. The molecular emission was mapped using the H-2 upsilon = 1 --> 0 S(1) line at 2.122 mum. The far-infrared (FIR) observations were obtained using ISOPHOT on the Infrared Space Observatory. The Halpha observations are more sensitive than previous measurements and show the huge extent of the Helix, confirming it as a density-bounded nebula and showing previously unseen point-symmetric structures. The H-2 observations show that the molecular gas follows the distribution of molecular material shown in previous work. The molecular emission is conned to that part of the nebula seen in the classic optical image. Furthermore, comparison of the H-2 emission strength with time-dependent models for photodissociation regions (PDRs) shows that the emission arises from thermal excitation of the hydrogen molecules in PDRs and not from shocks. The FIR observations, at 90 and 160 mum, represent mostly contributions from thermal dust emission from cool dust grains but include a small contribution from ionized atomic lines. Comparison of the FIR emission with the Halpha observation shows that the dust and ionized gas are coincident and extend to similar to1100" radius. This equates to a spatial radial extent of more than 1 pc (assuming a distance to the Helix of similar to200 pc). Assuming that the outer layers of the circumstellar shell have spherical symmetry, radiative transfer modeling of the emission in Halpha gives a shell mass of similar to1.5 M.. However, the modeling does not cover the outermost part of the shell (beyond similar to600" radius), and therefore this is a lower limit for the shell mass. Moreover, the models suggest the need for very large dust grains, with similar to80% of the dust mass in grains larger than 3.5 mum. Comparison of these new observations with previous observations shows the large-scale stratification of the Helix in terms of ionized gas and dust, as well as the coexistence of molecular species inside the ionized zones, where molecules survive in dense condensations and cometary knots.